If you're just using it, it isn't restrictive at all is it? You just have to say that you used it.

If you modify it, you must provide source code for the version that was distributed. Not the source code for anything else.

... unless I totally misunderstand LGPL

I am not a lawyer, but, yes, the LGPLv3 in particular is in fact quite restrictive relative to what is believed (LGPLv2 much less so).

The driving force behind the GPL-series of licenses is to maintain the GNU/Stallman freedoms [1], including "freedom to run the program as you wish, for any purpose" (0) and "to study how the program works, and change it so it does your computing as you wish" (1)

It is widely believed that any software implementing DRM on it's runtime code is incompatible with the (L)GPLv3, in particular signed firmware distributions or software distribution systems such as the Mac/iOS App Stores or Steam. The (L)GPLv3 was actually written with this in mind, with some of the authors calling it Tivoization [2] in reference to Tivo's locked down firmware.

The relevant legal jargon is in section 6 of the GPL [3], which the LGPL is built on top of, and states:

“Installation Information” for a User Product means any methods, procedures, authorization keys, or other information required to install and execute modified versions of a covered work in that User Product from a modified version of its Corresponding Source.

Again I am not a lawyer, but in other words, you must cough up your signing keys (which for Mac/iOS devs is incidentally a breach of your Apple Developer contract) in order to legally distribute signed software that uses LGPL libraries (-).

[-] It seems it should be OK to distribute non-DRM protected software, but for general consumer software this kind of distribution seems to be on it's way out in a hurry.

[1] http://www.gnu.org/philosophy/free-sw.html

[2] https://en.wikipedia.org/wiki/Tivoization#GPLv3

[3] http://www.gnu.org/licenses/gpl-3.0.en.html

I had no idea. So iOS devs cannot use LGPL software at all because of this?

LGPL v2.0/v2.1 do not have the "tivoization" clause, so it should be safe to use in iOS if you are following the rest of the license provisions.

LGPL v3 does, so it is likely not safe or legal to use that software in iOS or other drm scenarios.

You can find a lot of debate about this, I think mostly because people assume the two versions to be the same and do not specify which they are wanting to use.

Again I have no legal background and I recommend getting real legal advice as the issue is quite complex.